To remove toxic materials from exhaust gases discharged from internal combustion engines of automobiles, etc., ceramic honeycomb structures are used for exhaust-gas-cleaning catalyst converters, particulate matter (PM)-capturing filters, and carriers for catalysts for removing nitrogen oxides (NOx).
As shown in FIGS. 1(a) and 1(b), a ceramic honeycomb structure 1 comprises a ceramic honeycomb body 10 having large numbers of longitudinal cells 14 partitioned by porous cell walls 13, and a peripheral wall 11 formed on a periphery of the ceramic honeycomb body 10, with a substantially circular or ellipsoidal cross section perpendicular to its flow paths [see FIG. 1(a)]. The ceramic honeycomb structure 1 is assembled in a metal container (not shown), in which it is fixed in a holding member (not shown) formed by a metal mesh or ceramic mat, etc. Accordingly, the peripheral wall 11 should have enough isostatic strength to withstand heat shock in a state where the ceramic honeycomb structure 1 is held by the holding member.
To reduce the amount of nitrogen oxides (NOx) contained in exhaust gases of diesel engines, ceramic honeycomb structures carrying NOx catalysts on cell walls are used. To provide the ceramic honeycomb structures with higher NOx-cleaning capacity, it is effective to increase the amount of a catalyst carried. To this end, cell walls should have as high porosity as, for example, 50% or more.
JP 05-269388 A discloses a ceramic honeycomb structure comprising a ceramic honeycomb body having large numbers of longitudinal cells partitioned by porous cell walls, and longitudinal grooves open on the peripheral surface, and a peripheral wall formed by a coating material filled in the grooves. This ceramic honeycomb structure is produced by forming a sintered ceramic honeycomb body integral with a peripheral wall by a known method, removing peripheral cells by grinding to form a ceramic honeycomb body having grooves on the peripheral surface, filling the grooves on the peripheral surface with a coating material paste comprising ceramic particles and/or ceramic fibers and colloidal silica or colloidal alumina and drying it to form the peripheral wall. JP 05-269388 A describes that such method produces a ceramic honeycomb structure having a reinforced peripheral portion, with excellent heat resistance and heat shock resistance.
However, when the peripheral wall described in JP 05-269388 A is formed in a ceramic honeycomb structure comprising cell walls having as high porosity as, for example, 50% or more, the peripheral wall fails to exhibit a sufficient strength-improving effect because of extremely low strength of the cell walls, resulting in a ceramic honeycomb sintered body failing to having enough isostatic strength to withstand heat shock during use.
JP 2004-175654 A discloses a ceramic honeycomb structure comprising a ceramic honeycomb body having large numbers of longitudinal cells partitioned by porous cell walls and longitudinal grooves open on the peripheral surface, and a peripheral wall filling the grooves, with stress-releasing portions (voids) at least partially in the peripheral wall or between the peripheral wall and the grooves. JP 2004-175654 A describes that even if it were subject to heat shock, cracks due to heat shock would not easily propagate to cell walls, resulting in excellent heat shock resistance. This ceramic honeycomb structure is produced by forming a sintered ceramic honeycomb body integral with a peripheral wall by a known method, removing part of cell walls in the peripheral wall by machining to form a honeycomb body having grooves on a peripheral surface, applying a coating material comprising ceramic aggregate and an inorganic binder to substantially fill the grooves, and rapidly drying the coating material in a drying furnace at 70° C. or higher.
However, because the ceramic honeycomb structure described in JP 2004-175654 A has stress-releasing portions (crack-like voids open on an outer surface of the peripheral wall, or gaps between the ceramic honeycomb body and the peripheral wall), the peripheral wall is easily detachable from the ceramic honeycomb body. Particularly when the method described in JP 2004-175654 A is used for the production of a ceramic honeycomb body comprising cell walls having as high porosity as, for example, 50% or more, the resultant ceramic honeycomb body does not have sufficient isostatic strength.
JP 2006-255542 A discloses a ceramic honeycomb structure comprising a cellular structure having pluralities of cells partitioned by porous cell walls, and an outer wall formed by a coating material comprising ceramic particles having an average particle size of 20-50 μm on a peripheral surface of the cellular structure, the porosity of the outer wall being smaller in an outer region than in an inner region in a thickness direction. JP 2006-255542 A describes that the ceramic honeycomb structure has excellent durability and wear resistance because of little detachment of ceramic particles from the outer wall, and that printings on the outer wall surface are resistant to wear and damage. The ceramic honeycomb structure described in JP 2006-255542 A is produced by removing a peripheral portion from a honeycomb-structured sintered body formed by a known method by grinding, applying a coating material to the peripheral surface to form a peripheral coating layer, drying the peripheral coating layer completely or partially, and applying a coating material comprising colloidal ceramic such as colloidal silica, colloidal alumina, etc. as a main component to the peripheral coating layer to form a dense layer.
The peripheral wall described in JP 2006-255542 A, which has a porosity gradient, has excellent wear resistance and damage resistance in printings on the surface. However, when this peripheral coating layer is formed on a ceramic honeycomb body comprising cell walls having as high porosity as, for example, 50% or more, it is easily detached from the peripheral surface of the ceramic honeycomb body, because cell walls are extremely brittle. This means that the coated peripheral wall has poor adhesion to the ceramic honeycomb body.
JP 2003-284923 A discloses, as shown in FIG. 5, a ceramic honeycomb structure 50 comprising a ceramic honeycomb body 51, whose outermost cells and predetermined numbers of cells inside the outermost cells are cells 54 sealed by a peripheral wall 52 at one-side ends and/or in intermediate portions to prevent a fluid from flowing. JP 2003-284923 A describes that this ceramic honeycomb structure 50 can enhance the activity of a catalyst carried in a short period of time, because of a short temperature elevation time from the start of operation, which is achieved by the heat insulation of the sealed cells 54 formed by the peripheral wall 52. JP 2003-284923 A describes that this ceramic honeycomb structure 50 is produced by drying and sintering an extruded honeycomb-structured green body with different shrinkage ratios between two ends to form a frustoconical ceramic honeycomb body 51, machining a frustoconical peripheral surface of the ceramic honeycomb body to a cylindrical shape, and forming a peripheral wall 52 by a coating material such as a ceramic cement, etc. on a peripheral surface 51a thereof. JP 2003-284923 A lists cordierite, ceramic materials comprising cordierite and/or ceramic fibers and an amorphous oxide matrix (colloidal silica, colloidal alumina, etc.), etc., as materials for the peripheral wall 52.
However, when the invention described in JP 2003-284923 A is applied to a ceramic honeycomb body comprising cell walls having as high porosity as, for example, 50% or more, the peripheral wall described in JP 2003-284923 A cannot provide the ceramic honeycomb structure with sufficient isostatic strength, because cell walls are extremely brittle.